Apparatus for improving the signal information in the examination of samples by scanning electron microscopy or electron probe microanalysis

ABSTRACT

In the analysis of a sample bombarded by a scanning primary electron beam, in order to ensure that only low-energy electrons emitted by the sample contribute to the image formation in an evaluating apparatus, adjacent the sample face an alternating electric or magnetic field is generated which periodically weakens the current of electrons emitted by the sample so that a modulated electron flow reaches a secondary electron detector disposed in the vicinity of the sample.

O United States Patent [1 1 [in 3,714,424 Weber 1 Jan. 30, 1973 [54] APPARATUS FOR IMPROVING THE [56] References Cited SIGNAL INFORMATION IN THE EXAMINATION OF SAMPLES BY UN'TED STATES PATENTS SCANNING ELECTRON MICROSCOPY 3,535,516 10 1970 Munakata .;.'.250 49.5 A OR ELECTRON PROBE MICROANALYSIS Primary ExaminerArchie R. Borchelt [75] Inventor: Ulrich Weber, Karlsruhe, Germany Ass'smm Examfner c' Church Att0rneyEdwm E. Grelgg [73] Assignee: Siemens Aktiengesellschaft,

Munich, Germany 57 ABSTRACT [22] Filed: March 1971 In the analysis of a sample bombarded by a scanning [21] A N 122,785 primary electron beam, in order to ensure that only low-energy electrons emitted by the sample contribute to the image formation in an evaluating apparatus, ad- [30] Foreign Application Pnomy Data jacent the sample face an alternating electric or mag- March 10, I970 Germany P 20 ll 193.7 netie field is generated which periodically weakens the current of electrons emitted by the sample so that a [52] US. Cl. ..250/49.5 A, 250/4 9.5 PE modulated electron flow reaches a secondary electron [51] Int. Cl ..II0l 37/26 detector di in the vicinity of the Sample. [58] Field of Search ..250/49.5 A, 49.5 PE

SHEET 10F 2 PAIENIED JAN 30 I973 Fig.1

PAIENTED JAN 30 I973 SHEET 2 [IF 2 flns' k I Fig. 2

APPARATUS FOR IMPROVING THE SIGNAL INFORMATION IN THE EXAMINATION OF SAMPLES BY SCANNING ELECTRON MICROSCOPY OR ELECTRON PROBE MICROANALYSIS BACKGROUND OF THE INVENTION This invention relates to an apparatus associated with the scanning electron microscopy and the electron probe microanalysis wherein the surface of a sample face is scanned by means of a primary electron beam, and the electrons emitted upon bombardment with the primary electron beam are sensed by means of a secondary electron detector.

By means of a scanning electron microscope the outer configuration (topography) of a sample may be analyzed, while with the aid of an electron probe microanalyzer an elementary analysis of the sample surface may be performed. The results of these analyses may be plotted by means of an evaluating device or may be represented as an image. In both analyzing devices it is a requirement to detect with great sensitivity even very small structural differences in a sample face and to furnish contrast-rich images with optimal resolution.

In both types of analyzers a magnetic deflecting system is incorporated in the electro-optical system of the electron gun. The magnetic deflecting system permits by means of linewise deflection of the primary electron beam over the sample face and by means of synchronously controlled evaluating devices (registering devices, data supply devices, plotters, image forming apparatus, etc.) the measuring and representation of the course of signal intensity along lines or over a range of the sample face. Besides an analysis of the sample face a dot analysis of the sample can also be performed.

Upon impingement ofa high energy primary electron beam (5-40 keV) on a sample face, mostly X-rays, back-scattered electrons, secondary electrons and Auger electrons are emitted. In the scanning electron microscopy in the first place, low-energy (approximately 50 eV max.) secondary electrons are used as measuring signals. In the electron probe microanalysis, in contradistinction, practically all physical occurrences are used as possible sources of information for the analysis.

If the surface structure ofa sample is to be examined by means of either one of the afore-outlined analyzers, it is sought to utilize for the image formation only 'the low-energy secondary electrons emitted by the sample as a result of its bombardment with the primary electron beam, while the backscattered electrons should not be used. The reason therefor is that the dot resolution in the analysis of a surface of a sample having a large mass is by one order of magnitude better when performed with the aid of secondary electrons than if backscattered electrons are used.

It is known that scanning electron microscopes which, for the purpose of an electro-optical representation of microstructures of a sample face at extreme magnification and resolution, scan the surface with an extremely thin primary electron beam in a linewise manner for the image formation, utilize, among others, a so-called secondary electron detector. The latter indicates the detection of secondary electrons, emitted by the sample surface, by means of an amplified current signal which is a function of their number.

A secondary electron detector of this type comprises an electro-optical system and an indicating system. The electro-optical system has the function to deliver to the detector as many secondary electrons as possible as they leave in all directions the locus of impingement of the primary electron beam on the sample face. In. the indicating system the relatively slow secondary electrons are first applied to a post-accelerator system which accelerates them to an energy sufficiently high (approximately l-l 5 keV) for increasing the sensitivity of indication. The accelerated electrons enter an electron multiplier or a scintillation crystal after which there is connected a sensitive photomultiplier or a semiconductor detector.

A secondary electron detector necessarily responds also to the rapid backscattered electrons which are emitted from a larger surrounding area of the locus of impingement of the primary electron beam on the sample face. The signal of the backscattered electrons limits the image resolution and causes the image contrast to deteriorate. If the sensitivity of indicating backscattered electrons by the secondary electron detector is decreased, then a loss of sensitivity with respect to the secondary electrons has also to be taken into account.

A further deterioration of the resolution and the image contrast is caused by the tertiary electrons which are also sensed by the secondary electron detector. Tertiary electrons are secondary electrons which are emitted by adjacent components (such as the pole piece of the electron lens assembly, or the walls of the sample chamber) upon bombardment by the backscattered electrons. Under unfavorable examining conditions, the image signal formed by the parasite electrons, that is, by the backscattered and the tertiary electrons, may be stronger than the image signal of the secondary electrons.

OBJECT AND SUMMARY OF THE INVENTION It is an object of the invention to provide an apparatus for the scanning electron microscopy and the electron probe microanalysis which, on the one hand,

,prevents the participation in the image formation of apparatus for the periodic weakening or interruption of the current oflow-energy electrons emitted by the sample face and entering the secondary electron detector. There is further provided an image signal emitter which is tuned to the frequency of the modulating apparatus and which is connected after the secondary electron detector.

The invention will be better understood as well as further objects and advantages of the invention will become more apparent from the ensuing detailed specification of two exemplary embodiments taken in conjunction with the drawing.

BRIEF DESCRIPTION CF THE DRAWING FIG. 1 is a schematic representation, including a circuit diagram, of a first embodiment of the invention; and

FIG. 2 is a schematic representation, including a circuit diagram, of a second embodiment of the invention.

' DESCRIPTION OF THE EMBODIMENTS Turning now to FIG. 1, a primary electron beam E passes through an electron lense L and is deflected to scan a sample P. The electronic deflecting apparatus including an associated control system is of conventional nature and is not illustrated. The sample I and the electron lense L are surrounded by a sample chamber wall W. The normal of the sample face is moved from its position parallel to the primary electron beam E through an adjustable angle a in the direction ofa secondary electron detector D known by itself.

The first output terminal XI of an oscillator is connected with the sample P, whereas the second output terminal K2 is connected with the sample chamber wall W. Both the sample chamber wall W and the electron lense L are grounded (i.e. they are at zero potential). The oscillator O delivers an alternating voltage of a frequencyfand generates an alternating electric field in the order of V/cm in a condenser, one electrode of which is formed by the sample P, while the other electrode of which is formed by a component adjacent the sample P, such as the sample chamber W and the electron lense L. It is to be noted that the second output terminal K2 of the oscillator 0, instead of connecting it to components W and L may be connected to an auxiliary electrode disposed in the space in front of the sample face. The auxiliary electrode may be an annular disc or may have a pointed, or edge-like or sieve-like configuration.

The secondary electron detector D, the high voltage supply of which is not indicated in the figure, applies to the input terminals El and E2 of an image signal emitter such as a phase-responsive rectifier (lock-in amplifier) PG, a voltage which is proportionate to the number of electrons entering its inlet opening F. The input terminal E2 is grounded. For applying a reference signal, additional input terminals L1 and L2 of the phase-responsive rectifier PG are connected with the output terminals KI and K2 of the oscillator 0 through a potentiometer RI. The phase-responsive rectifier PG preferably contains a phase adjusting component. The image signal S delivered by the phase-responsive rectifier PG is applied to a known evaluating apparatus A, such as an image forming device controlled synchronously with the scanning motion of the primary electron beam E.

At the sample face, in addition to the alternating field, there is generated a direct electric field ofa magnitude in the order of I00 L000 V/cm. For this purpose, a voltage source B1 of approximately 200 V is connected between the inlet opening F of the secondary electron detector D and the components adjacent the sample P. The positive pole of the voltage source B1 is applied to the inlet opening F, whereas its negative pole is grounded. It is to be understood that for some purposes a reversal of the poles may be expedient. The purpose of this electric field is to securely discriminate between the electrons entering the detector opening F and to obtain at the output of the secondary detector D an as large an output signal as possible.

Further, for reasons of the penetration of electrical force lines on the sample P, a further electric DC field of adjustable magnitude and direction may be generated at the sample face. For this purpose, the center tap of a further potentiometer R2, which is connected to a further voltage source B2, is connected with the sample P. A reversing switch U is provided to reverse the polarity of the potential applied to the sample P. One pole of the voltage source Bl is always grounded. According to the example shown in FIG. 1, the sample P is at a negative potential (for example 5 V) with respect to its environment (W, L). The direction of this electric field is set by adjusting the angle 0: and/or by actuating the reversing switch U,

while its intensity is set by the potentiometer R2.

The high-energy (for example, 20 keV) beam E of the primary electrons causes, at the bombarded location of the sample face, an emission of high-energy (approximately 20 keV) backscattered electrons e,., lowenergy (approximately up to 50 eV) secondary electrons e, and also, in a smaller number, low-energy Auger-electrons (not shown). In FIG. 1 conditions are shown at a moment when the negative amplitude (for example -10 V) of the alternating voltage supplied by the oscillator O is applied to the sample P, the potential of which is then l5 V with respect to the ground. The backscattered electrons e, emitted by the sample face leave the latter in linear paths since, due to their high kinetic energy, they are unaffected by the momentarily positive potential (in the example a total of 215 V) of the detector opening F. A small portion of the backscaattered electrons e, enters the detector D. The low-energy secondary electrons e,, on the other hand, are deflected along curved paths in the direction of the detector opening F. Some of the low-energy tertiary electrons e, emitted by adjacent components (such as the electron lense L or the wall W of the sample chamber) upon their bombardment by the backseattered electrons 2, also enter the detector opening F.

After one half period, to the sample P there is applied a positive amplitude (for example l0 V) of the alternating voltage supplied by the oscillator 0. At that instant the potential of the sample P is +5 V with respect to the ground. The rapid backscattered electrons e,- are, as before, unaffected by the potential (now I V) applied to the detector opening F. Now again, a small portion of the back-scattered electrons e enters the detector D. The tertiary electrons e because they are generated by the backscattered electrons e,., are little affected by the change of potential of the sample P.

On the other hand, a substantial portion of the secondary electrons e, is lost for sensing by the detector D, because the electric 1 field immediately adjacent the sample face was decreased. The largest part of the secondary electrons is now altogether incapable of leaving the sample face since in front thereof a space charge buildup has taken place.

It is thus seen that the periodic variation of the electric field between sample P and the adjacent components affects mostly the secondary electrons e,: they are periodically and in an alternating manner withheld or accelerated. In the phase responsive rectifier PG only that component of the output voltage of the secondary electron detector D is amplified and rectified which varies periodically and in phase with the frequencyfof the oscillator O. The image signal S thus contains mostly that information pertaining to the structure of the sample face which is carried by the secondary electrons e,.

Turning now to FIG. 2, in the embodiment shown therein, the current of low-energy electrons entering the secondary electron detector D, is weakened periodically by means of an alternating magnetic field having a magnitude between 1 and 100 oersteds.

For the generation of such a magnetic field, at each side of the tilted sample P, in the space between the latter and the electron lense L there are arranged two halves (only one shown) ofa coil C. To the latter there is applied an alternating current of frequencyfsupplied by an oscillator O. Expediently,'the latter is a sine wave or square wave generator. The force lines of the magnetic field extend practically parallel to the surface of the sample P. The low-energy electrons emitted by the sample upon bombardment by the primary electron beam E that scan the surface of the sample P are exposed to a force component of the magnetic field. This force component acts in the image plane in a direction normal to the primary electron beam E.

To the secondary electron detector D arranged laterally to the sample P, there is connected a narrow bandpass filter F which is tuned to the frequencyfof the oscillator O and after which there is connected an amplifier V. The output signal of the latter is applied, as an image signal S, to an evaluating device A and is plotted synchronously with the scanning motion of the primary electron beam E. It is noted that the fields should be so directed and their intensity so chosen that they have negligible or no effect on the primary electron beam.

The advantages achieved by means of the invention reside particularly in the fact that in forming the image of a sample surface optimal resolution and extremely differentiated image contrast may be achieved since exclusively electrons of low energy, mostly secondary electrons, take part in the image formation. Furthermore, contrary to the conventionally used secondary electron detectors, an energy analyzer is no longer necessary for keeping the backscattered electrons away from the detector. Such a measure has led to a loss of sensitivity. Further, by practicing the invention, the detector opening F no longer has to be brought into a geometrical position to render impossible the entry of backscattered electrons. Instead, the secondary electron detector may be oriented towards the sample in the immediate vicinity thereof even if the sample face is normal (or to the bombarding primary electron beam. In the latter case an alternating electric field of high amplitude may be generated at the sample face even with a low alternating voltage. During the half period when the negative amplitude is applied to the sample, there results an increase in the number of secondary electrons that reach the detector.

What is claimed is:

1. In an apparatus for analyzing a sample, said apparatus being of the type that includes a secondary electron detector having an inlet opening receiving high-energy and low-energy electrons emitted from a face of said sample upon bombardment with a scanning primary electron beam, the improvement comprisirfig Id modulating means generating an alternating at said sample face for effecting, with a determined frequency, a periodic decrease solely of the current of said low-energy electrons entering the inlet opening of said secondary electron detector; the possible range of said periodic decrease includes an interruption of said current and B. an image signal emitter connected to said secondary electron detector to receive the output signals of the latter, said image signal emitter being coupled to said modulating means for tuning said image signal emitter to the frequency of said modulating means.

2. An improvement as defined in claim 1, wherein said modulating means includes means for generating an alternating electric field at the sample face from which said electrons are emitted.

3. An improvement as defined in claim 2, including an oscillator having a first output terminal connected to said sample and a second output terminal connected to electrode means disposed adjacent said sample face.

4. An improvement as defined in claim 1, wherein said modulating means includes means for generating an alternating magnetic field at the sample face from which said electrons are emitted.

5. An improvement as defined in claim 4, including A. an oscillator having output terminals and B. an electric coil disposed adjacent said sample and connected to said output terminals.

6. An improvement as defined in claim 1, wherein said image signal emitter includes A. an amplifier and B. a narrow bandpass filter connected to and before said amplifier and tuned to the frequency of said modulating means.

7. An improvement as defined in claim 1, including A. a phase-responsive rectifier constituting said image signal emitter and B. an oscillator having an output connected to an input of said phase-responsive rectifier to apply a reference frequency to the latter.

8. An improvement as defined in claim 1, including means to generate a constant, adjustable electric field between said inlet opening of said secondary electron detector and said face of said sample. 

1. In an apparatus for analyzing a sample, said apparatus being of the type that includes a secondary electron detector having an inlet opening receiving high-energy and low-energy electrons emitted from a face of said sample upon bombardment with a scanning primary electron beam, the improvement comprising A. modulating means generating an alternating field at said sample face for effecting, with a determined frequency, a periodic decrease solely of the current of said low-energy electrons entering the inlet opening of said secondary electron detector; the possible range of said periodic decrease includes an interruption of said current and B. an image signal emitter connected to said secondary electron detector to receive the output signals of the latter, said image signal emitter being coupled to said modulating means for tuning said image signal emitter to the frequency of said modulating means.
 1. In an apparatus for analyzing a sample, said apparatus being of the type that includes a secondary electron detector having an inlet opening receiving high-energy and low-energy electrons emitted from a face of said sample upon bombardment with a scanning primary electron beam, the improvement comprising A. modulating means generating an alternating field at said sample face for effecting, with a determined frequency, a periodic decrease solely of the current of said low-energy electrons entering the inlet opening of said secondary electron detector; the possible range of said periodic decrease includes an interruption of said current and B. an image signal emitter connected to said secondary electron detector to receive the output signals of the latter, said image signal emitter being coupled to said modulating means for tuning said image signal emitter to the frequency of said modulating means.
 2. An improvement as defined in claim 1, wherein said modulating means includes means for generating an alternating electric field at the sample face from which said electrons are emitted.
 3. An improvement as defined in claim 2, including an oscillator having a first output terminal connected to said sample and a second output terminal connected to electrode means disposed adjacent said sample face.
 4. An improvement as defined in claim 1, wherein said modulating means includes means for generating an alternating magnetic field at the sample face from which said electrons are emitted.
 5. An improvement as defined in claim 4, including A. an oscillator having output terminals and B. an electric coil disposed adjacent said sample and connected to said output terminals.
 6. An improvement as defined in claim 1, wherein said image signal emitter includes A. an amplifier and B. a narrow bandpass filter connected to and before said amplifier and tuned to the frequency of said modulating means.
 7. An improvement as defined in claim 1, including A. a phase-responsive rectifier constituting said image signal emitter and B. an oscillator having an output connected to an input of said phase-responsive rectifier to apply a reference frequency to the latter. 